The present invention relates to the pressure compacting or molding of settable gypsum/plaster compositions, and to the shaped articles resulting therefrom, including modular constructional units or building materials, and other load-bearing members, which shaped articles are characterized by excellent mechanical properties and enhanced water repellency.
In one embodiment disclosed and claimed in my aforenoted copending '492 application, there is described the pressure compacting of mixtures essentially consisting of plaster and "wet" gypsum, and wherein the plaster has a Blaine surface of between 500 and 15,000 cm.sup.2 /g, the "wet" gypsum contains between 15 and 40 parts by weight of water and has a Blaine surface of between 200 and 10,000 cm.sup.2 /g, and the percentage by weight of the plaster in the mixture of gypsum and plaster is between 30 and 60%, with the ratio by weight of plaster/gypsum being such that the excess of water with respect to that stoichiometrically required for ultimate complete setting of the mix being between 0 and 15% of the total weight of the admixture.
In another embodiment disclosed and claimed in my said '492 copending application, there is described the pressure compacting of gypsum/plaster/water admixtures, wherein the gypsum contains from 0 to 40% by weight of free water, the percentage by weight of the plaster in the mixture of gypsum and plaster being comprised between 60% and 99%, and the total amount of water being such that the free water content of the gypsum together with the added water represents an excess with respect to that amount stoichiometrically required for subsequent complete setting of the mixture of between 0 and 15% by weight with respect to the total weight of said mixture.
The shaped articles and load-bearing members which are obtained consistent with my Ser. No. 876,492 pressure compacting process display mechanical properties similar to those obtained with conventional prefabricated plaster elements, for example, bending strength of 25 to 35 kg/cm.sup.2 and a compressive strength of 60 to 80 kg/cm.sup.2, when utilizing the subject moist gypsum/plaster admixtures and operating under compacting pressures of less than 100 kg/cm.sup.2. Better results are obtained by increasing the weight of the mixture to be compacted and by increasing the compacting pressure, for example, bending strengths of up to 100 kg/cm.sup.2 and compressive strengths of up to 400 kg/cm.sup.2 are readily attained.
Such resultant shaped articles thus have excellent mechanical, thermal and acoustical insulating properties. It is known, however, that all plaster shaped articles are naturally hydrophilic and display the disadvantage that, if contacted with water, same progressively lose their mechanical properties, this being the reason that external applications of plaster are essentially only coatings oriented, with virtually no use thereof in the field of building materials and load-bearing elements.
Accordingly, there exists a great need in this art to improve the water repellency of gypsum/plaster shaped articles, including those described in my said '492 copending application.
Various attempts have to date been made to render plaster water repellent. Compare the British Pat. Specification No. 380,321; same suggests the application to the surface of plaster or concrete materials of an aqueous solution of a soluble salt of a fatty acid, such as the potassium salt; the salt selected may be applied in solution or in emulsion, and, after drying, provides a water repellent surface.
A suspension-based waterproofing technique has also been proposed, in the French Pat. Application No. 2,345,407. By means of this particular technique, all of the material and not merely its face surfaces is rendered water impermeable, by incorporating during the compounding of the plaster with the water a sealing emulsion comprising asphalt, a wax, a borate and polyvinyl alcohol. The plaster treated in this manner with a water emulsion provides, after setting and drying, certain building materials such as plaster squares.
Hence, it is apparent that the prior art has not succeeded in providing water repellent materials based on plaster by an economically viable process. Frequently, use is made of a waterproofing agent in emulsion or solution; this, however, results in such problems as adequate distribution of the agent throughout the plaster and the necessity for a drying stage.